The present invention relates to a hydraulic boosting device of open centre type such as a brake booster of open centre type which intensifies braking force by boosting leg-power exerted on a brake pedal in a vehicle by hydraulic fluid, and more particularly to a hydraulic boosting device provided with an emergency accumulator in which emergency fluid pressure is stored for actuating the hydraulic boosting device when fluid pressure of hydraulic fluid is dropped.
For example, a hydraulic boosting system which boosts operating force exerted on an operating member by using the fluid pressure of hydraulic fluid is sometimes used in a vehicle. As one of such hydraulic boosting systems, there is a hydraulic brake system in which a hydraulic boosting device which operates with fluid pressure of hydraulic fluid is employed. The hydraulic boosting device boosts leg-power exerted on a brake pedal to actuate a master cylinder with the boosted power in order to provide large braking forces which can not be obtained by the leg-power alone or to reduce leg-power to be exerted on the brake pedal.
One of such conventional hydraulic brake systems is a hydraulic brake system as shown in FIG. 11. In this figure, reference numeral 1 designates the hydraulic brake system, 2 designates a brake pedal, 3 designates an open centre type hydraulic brake booster (hereinafter, sometimes referred to as "brake booster" or just "booster") which is driven by the brake pedal 2 to boost leg-power exerted on the brake pedal 2, 4 designates a tandem master cylinder which is actuated by the output of the booster 3 to develop brake fluid pressures, 5 designates brake cylinders which are actuated by the brake fluid pressures from the master cylinder 4 to produce braking forces for respective wheels, 6 designates a pump which is driven by an engine 7 to send hydraulic fluid to the booster 3, 8 designates a reservoir in which the hydraulic fluid is stored, and 9 designates an emergency accumulator in which an emergency fluid pressure for actuating the booster 3 is stored for actuating the booster 3 when the pump 6 fails and thus no hydraulic fluid is not supplied from the pump 6.
The open centre type booster 3 allows the free flow of the hydraulic fluid by maximizing a space of a control valve when braking operation is not performed and restricts the flow of the hydraulic fluid to develop a fluid pressure by throttling the control valve when the braking operation is performed and outputs the fluid pressure. Several types of such boosters have been known. One of such boosters (booster 3) is shown in FIG. 12. Since the booster 3 has been known in the art and a booster of the present invention will be described in detail later, the description will be made as regard to only parts relating to problems to be solved by the present invention.
FIG. 12 shows the booster in an inoperative state i.e. when the braking operation is performed. In this state, a space between a first annular groove 10 and a second annular groove 11 is maximized, the communication between the second annular groove 11 and the third annular groove 12 is interrupted, the third annular groove 12 and the fourth annular groove 13 communicate. Therefore, hydraulic fluid discharged from the pump 6 is returned to the reservoir 8 through an inlet path 14, the second annular groove 11, the space between the first annular groove 10 and the second annular groove 11, the first annular groove 10, and a circulating path 15 of the open centre type booster 3. Since the space between the first annular groove 10 and the second annular groove 11 is maximized in this case, little fluid pressure is developed in the circulating hydraulic fluid.
As the input shaft 16 moves forward by pedaling the brake pedal 2 in this state, a pair of levers 17, 18 (which are superimposed in a direction perpendicular to a surface of the drawing in FIG. 12) pivot so that the valve spool 19 moves forward. Then, the space between the first annular groove 10 and the second annular groove 11 is restricted, the communication between the second annular groove 11 and the third annular groove 12 is allowed, and the communication between the third annular groove 12 and the fourth annular groove 13 is interrupted. Because the space between the first annular groove 10 and the second annular groove 11 is restricted (the space sometimes finally becomes 0) a fluid pressure is developed in the second annular groove 11. The fluid pressure is introduced into a power chamber 23 through the space between the second annular groove 11 and the third annular groove 12, a first radial hole 20, an axial hole 21, a third check valve, and second radial holes 22 and is then exerted on a power piston 24. As a result of this, the power piston 24 produces brake operating force which is the boosted leg-power. The brake operating force is outputted by an output shaft 25 and thus operates the master cylinder 4 to actuate brakes.
The fluid pressure developed in said annular groove 11 moves a valve body 29 in a charging valve 28, which consists of a check valve, of an accumulator valve 27 to the right in FIG. 12 so as to part the valve body 29 from a rubber seat 30 to open the charging valve 28. Therefore, the fluid pressure is introduced into the emergency accumulator 9 through a space between the valve body 29 and the rubber seat 30, the periphery space of the valve body, and an accumulator path 31 and is stored in the emergency accumulator 9.
By releasing the brake pedal 2, the input shaft 16 and the valve spool 19 retreat in the inoperative position as shown in FIG. 12 so that the communication between the third annular groove 12 and the fourth annular groove 13 is allowed, the communication between the second annular groove 11 and the third annular groove 12 is interrupted, and the space between the first annular groove 10 and the second annular groove 11 becomes maximum. Therefore, the hydraulic fluid in the power chamber 23 is discharged into the reservoir 8 through the holes 22, 21, 20, the third annular groove 12, the space between the third annular groove 12 and the fourth annular groove 13, the fourth annular groove 13, and a discharge path 32. As a result of this, the power piston 24 retreats to the inoperative position and thus the brake operating force becomes extinct so that the master cylinder 4 returns in the inoperative state and thus the braking operation is canceled. Since the space between the first annular groove 10 and the second annular groove 11 becomes maximum, the fluid pressure developed in the second annular groove 11 becomes extinct. The numeral 77 designates a relief valve which opens to relieve the fluid pressure in the emergency accumulator 9 to the path 14 when the pressure stored in the emergency accumulator 9 exceeds a predetermined pressure.
When the pump 6 fails so that no fluid pressure is developed even when the space between the first annular groove 10 and the second annular groove 11 is restricted, the valve spool 19 moves forward and reaches the full-stroke point by further pedaling the brake pedal 2 largely. After the valve spool 19 reaches the full-stroke point, the brake pedal 2 is further pedaled so that the input shaft 16 further moves forward. Accordingly, the levers 17, 18 further pivot and the slide valve 33 moves forward relative to the valve spool 19. Then, the second radial holes 22 are closed so that the power chamber 23 is shut off from the pump 6. As the slide valve 33 further moves forward, a retainer 34 moves forward. A valve body 35 of a dump valve 36, which consists of a check valve, in the accumulator valve 27 is moved forward by the retainer 34 so that the dump valve 36 opens. Thus, the fluid pressure stored in the emergency accumulator 9 is introduced into the power chamber 23 whereby the power piston 24 operates. Therefore, even when the pump 6 fails, the brakes can be actuated because the leg-power is boosted by the fluid pressure in the emergency accumulator 9 during the predetermined pressure is stored in the emergency accumulator 9.
By the way, in the hydraulic brake system 1, the pressure storage to the emergency accumulator 9 is performed by introducing the fluid pressure developed during the braking operation, i.e. the discharge pressure of the pump, into the emergency accumulator 9. Therefore, when the brake operating force is small, the pressure storage to the emergency accumulator 9 is not sufficiently performed. In this case, when the pump 6 fails, the potential times that the booster can boosts the leg-power with the fluid pressure in the emergency accumulator 9 is reduced and thus the sufficient braking forces can not be provided.